As concerns for the environmental costs related to the use of non-renewable resources grow, energy production based on alternative resources such as solar, wind, and biomass is being developed. Unfortunately, the relative ease of energy production and the high energy density available in non-renewable resources is difficult to match. For instance, solar power requires large surface area collectors and wind power requires large wind turbines. In addition, both of these routes require energy storage systems, which add additional energy conversion steps and associated efficiency losses.
Use of biomass to replace nonrenewable energy sources has shown great promise, such as in the formation of biofuels and industrially useful bioplastics. The relatively low energy density of biomass as compared to fossil fuels such as coal and natural gas remains a problem for large-scale, high energy intensity use of biomass, for instance as an energy source for power plants.
Lignocellulosic biomass is a most readily available renewable natural resource and provides the two most abundant organic compounds on earth in lignin and cellulose, with the other major components including hemicellulose. The usefulness of cellulose has long been recognized, primarily in paper making, but increasingly for use in formation of biofuels due to the high energy density. Lignin is a generic term for a complex polymer of aromatic alcohols that varies somewhat between plant species, but in all forms has a very high energy density. It is an integral part of the cell wall that covers and protects the cellulose and hemicellulose with such efficiency that it also presents many challenges to the successful use of the biomass.
Attempts have been made to utilize lignocellulose biomass in energy production through torrefaction and densification. Torrefaction is a mild form of pyrolysis at relatively low temperature in a low oxygen atmosphere that increases the energy density of a lignocellulose biomass. During torrefaction, water may evaporate and volatile organic compounds may decompose and gassify, resulting in a loss of mass and chemical energy in the gas phase. However, because more mass than energy is lost, torrefaction results in energy densification, yielding a torrefied product with lower moisture content and higher energy content compared to untreated biomass. To further increase the energy density, torrefied biomass has been compacted to a highly dense form.
Unfortunately, torrefied and densified biomass formed to date has fallen short of the energy density and bulk density necessary for use as a high energy fossil fuel replacement. For instance, while the densified products have shown increase in energy density as compared to the starting materials (though not equal to fossil fuels), they still cannot be handled during shipping, storage, and use without excessive breakdown and fines formation. In addition, they cannot be ground in a ball mill to a size suitable for use, e.g., in coal-fired boilers. Moreover, the densified materials maintain a high level of volatile organic compounds, and release volatiles at relatively low temperatures above about 90° C.
What are needed in the art are methods and systems that can be utilized to form a high energy density and high bulk density biomass. The high energy and bulk density biomass can be beneficially utilized as a replacement for fossil fuel based energy sources such as coal.